The present invention relates to compensating for polarization mode dispersion in a fiber transmission system. More specifically, the present invention relates to compensating for higher order polarization mode dispersion and in one illustrative embodiment the compensation is performed at the front end of the fiber transmission system.
Optical telecommunications generally involves the use of light modulated with data, transmitted through optical fibers. As the light propagates through the fiber, its signal characteristics may become distorted by the fiber in a number of ways. One type of distortion is known as polarization mode dispersion, or xe2x80x9cPMDxe2x80x9d.
PMD refers to an effect that an optical device, such as a span of optical fiber, has on the separate polarizations of a light beam. A light beam can be approximated as having electrical components that vibrate at right angles to the direction of travel. In the simple case the polarization or state of polarization of the light beam can be thought of as the direction of these right angle vibrations. In the more general case, these components are superimposed in a more complex way. As shown in FIG. 1, within a short optical fiber section 10, an orthogonal set of two polarized waveguide modes 20 and 30 can be found which have electric field vectors aligned with the symmetry axes of the fiber. The polarization of a light beam propagating through the fiber section can be represented by the superposition of vector components aligned with these polarization waveguide modes of the fiber as shown in FIG. 2. In FIG. 2, the polarization waveguide modes 20 and 30 are shown as two axes. The input polarization 40 is represented as the vector sum of two components 50 and 60 which are aligned with the polarization waveguide modes of the fiber section.
In ideal fiber, which has a perfect circular cross-section and is free from external stresses, the propagation properties of the two polarized waveguide modes are identical. However, imperfections introduced in the manufacturing process may result in fiber that is not perfectly circular. In addition, fiber that has been installed may suffer from external stresses arising from pinching or bending. These manufacturing imperfections and external stresses cause the two polarized waveguide modes to have different propagation characteristics which in turn gives rise to polarization mode dispersion, or xe2x80x9cPMDxe2x80x9d.
A convenient way to represent the effects of PMD caused by a particular optical device or span of optical fiber uses Stokes space, a three-dimensional geometrical space, and the Poincarxc3xa9 sphere, a sphere within Stokes space where every possible polarization state maps to a specific (and different) point on the sphere. Three axes, S1, S2, and S3, define this three dimensional space and any polarization can be described in reference to these axes, in other words by its S1, S2, and S3 components. The S1, S2, and S3 components of a polarization are called its Stokes parameters.
PMD affects the propagation of a light beam with respect to both time and frequency. With respect to time, PMD causes the two vector components comprising the polarization of the light beam to propagate down the two polarization waveguide modes at different velocities and thus separate in time as seen in FIG. 3. In FIG. 3, the two components 50 and 60 of input polarization 40 are aligned with polarization waveguide modes 20 and 30. This time gap is known as the differential group delay, xe2x80x9cDGDxe2x80x9d, represented as xcex94xcfx84. The difference of arrival times of the two pulses results in a broadening of the combined signal. The larger the differential group delay the broader the combined pulse. This in turn restricts the bit rate that can be transmitted through the fiber. With respect to frequency, the output polarization will vary as a function of the optical carrier frequency in a periodic fashion when the polarization of the light beam at the input remains fixed.
In general, an optical fiber will not have identical imperfections such that the two polarized wave-guide modes have the same orientation along the entire length of the fiber. However, most fibers can be modeled as a concatenation of many smaller fiber sections, each of which is considered to have a uniform birefringence and thus impart a uniformly oriented PMD to the light beam travelling through it. Birefringence refers to the difference in indices of refraction or light velocity of the two polarization components of a fiber. The effect over the full span is analyzed by considering the smaller lengths to be joined such that their respective axes are oriented at random angles relative to each other.
Although the behavior of a real length of fiber is more complex than that of a small section, over a narrow frequency range the PMD effects of both the real length and simple short length fibers are similar. However, instead of two polarization waveguide modes, the real length of fiber can be viewed as having orthogonal pairs of special polarizations, called the principal states of polarization (xe2x80x9cPSPxe2x80x9d) which, in general, vary with frequency. When a pulse is launched into a fiber with some optical power on each PSP, the output will consist of two light pulses separated in time by the differential group delay. In the absence of polarization-dependent loss, each optical device or span of fiber has a different orthogonal pair of PSP""s for each frequency. Polarization dependent loss refers to the difference in the amount of loss a light wave can experience with changes in its state of polarization.
As stated above, even for a simple short fiber section, PMD causes the polarization of the light beam at the output of the fiber section to vary with frequency. The frequency effect of PMD can be easily seen when displayed on the Poincarxc3xa9 sphere. As shown in FIG. 4, for a light beam having a fixed input polarization 40, the output polarization 70 of the light beam moves locally in a circle on the surface of the Poincarxc3xa9 sphere as the frequency of the light beam is varied from xcfx891 to xcfx892 to xcfx893.
Using Stokes space and the Poincarxc3xa9 sphere, the various effects of PMD for a given optical device or span of fiber may be compactly represented using a single, three-dimensional vector referred to as the PMD vector or xcexa9. The magnitude of the PMD vector, |xcexa9|, describes the time effect of PMD and the rate of rotation of the output polarization with respect to frequency. In other words, |xcexa9|=xcex94xcfx84. The direction of the PMD vector points along one of the PSPs for the fiber. This can be represented mathematically as xcexa9=xcex94xcfx84q, where q is the unit vector indicating the direction of one of the PSPs.
Since PMD can limit the transmission bandwidth of optical fiber, measurement of the PMD of a span of fiber is necessary to determine the span""s data transmission capability as well as to provide information for compensating the PMD in the span. There are many known methods for measuring PMD. Some methods only provide a measurement of the magnitude of PMD, i.e., the differential group delay, and do not provide information on the PMD vector characteristics. A method for measuring PMD vectors is the Poincarxc3xa9 Sphere Technique, or xe2x80x9cPSTxe2x80x9d. For each PMD determination, two different input polarizations are injected into an optical device under test, such as a fiber section, at each frequency of a frequency pair and the output polarizations are measured. Specifically, a light beam having a first input polarization is injected at the first frequency of the frequency pair into an optical device under test and the output polarization measured. Then, a light beam having this same first input polarization is injected at the second frequency of the frequency pair into the device under test and a second output polarization is measured. Third, a light beam having a second input polarization is injected at the same first frequency of the frequency pair into the device under test and a third output polarization is measured. Finally, a light beam having this same second input polarization is injected at the same second frequency of the frequency pair into the device under test and a fourth output polarization is measured. Depending on the results, a different first polarization may have to be chosen and the process repeated. The PMD for this first frequency pair is then calculated. This same procedure is used to determine the PMD for the other frequency pairs remaining in the frequency range being tested.
FIG. 5 shows a block diagram of a general apparatus capable of carrying out the previously described method. Control block 550, which could be a computer, directs tunable laser source 510 to sequentially emit light beams of various frequencies, such as the first and second frequencies described above. Control block 550 also directs polarizing device 520 to impart one of several polarizations to the beams emitted from 510, such as the first and second polarizations described above. Polarizing device 520 could consist of one or more linear or circular polarizers, with the number and type of polarizers depending upon the requirements of the specific PMD measurement method used. The light beams pass through the device being tested 530, such as a section of fiber, and are captured in polarization measuring device 540, which could be a polarimeter. Polarization measuring device 540 then measures the output polarization states of the light beams and passes this information to analysis block 560. Analysis block 560, which could be a computer, then calculates the PMD according to the algorithm used by the specific method.
Other methods have been proposed to determine the full vector characteristics of PMD for deducing the effects of higher order PMD. Higher order PMD describes the change of the PMD vector with frequency. Knowledge of the higher order PMD effects is necessary where there are significant changes of the PMD vector across the signal frequency bandwidth. Second order PMD specifically, reflects the linear frequency dependence of both the magnitude and direction of xcexa9. In other words, second order PMD manifests in a linear change of the differential group delay and PSPs with frequency.
This later set of methods of determining higher order PMD are described in U.S. patent application Ser. No. 09/395,238, filed on Sep. 13, 1999 and entitled Apparatus And Method For Improving The Accuracy Of Polarization Mode Dispersion Measurements, hereby incorporated by reference as if fully set forth herein, and U.S. patent application Ser. No. 09/390,033, filed on Sep. 3, 1999 and entitled Method For Measurement Of First- And Second Order Polarization Mode Dispersion Vectors In Optical Fibers. Both of these are commonly assigned with the subject invention.
The ability to measure PMD or its effects is one thing, to correct for PMD is another matter. Existing methods for compensating for PMD are known. However, they are all limited in scope. One such known method is described in Takashi Ono et al., xe2x80x9cPolarization Control Method for Suppressing Polarization Mode Dispersion Influence in Optical Transmission Systems,xe2x80x9d in Journal of Lightwave Technology vol. 12, No. 5 pp 891-897 (1994), hereby incorporated by reference as if fully set forth herein. Ono teaches that for a given optical frequency the differential group delay caused by PMD has no effect on the signal when the light beam is launched with a polarization that is aligned with one of the two input PSPs of the optical fiber. By launching at the PSP there is effectively only one propagation component. Therefore there can be no difference in velocity between the two components and this effect of PMD is avoided.
Ono however, does not eliminate the effects of second and higher order PMD. In a real optical system, a signal has a bandwidth or range of optical frequencies, while the signal is usually launched with a single polarization. Since the PSP of the optical fiber varies with frequency, the polarization of the light beam loses its alignment with the PSP across the signal bandwidth. This effect is not compensated by Ono""s method.
In accordance with the present invention first and higher order PMD in an optical fiber is compensated by aligning the polarization of an input light beam to the fiber, with one of the PSPs of the waveguide, regardless of the frequency of the light beam. The input light beam is first passed through a device which varies the polarization of the light beam with its change of optical frequency so that the polarization remains aligned with the PSP of the optical fiber. This device allows for higher order PMD compensation.
In one preferred embodiment, an adjustable device is used for transforming the polarization of the light beam as a function of frequency as required. Since the PMD of a fiber is a function of the frequency and the carrier frequency is different for different channels, it is necessary to adjust the rate of change of polarization of the transferred light beam according to the carrier frequency selected.
In another preferred embodiment applying known methods for chromatic dispersion compensation may also compensate the change in differential group delay arising from the change in frequency. Any one of known methods for chromatic dispersion compensation can be applied at the output of the optical fiber, in advance of a receiver.